Just installed Fedora 21 as a guest on a Windows 7. Some weird problems getting everything to work but a couple of things to remember:

• To get full screen working properly make sure you check “Auto resize Guest Display”, in the View menu of a running VM.
• Chromium based browsers (Chrome, Opera and Vivaldi) have been having some problems with mouse integration in VirtualBox, at least on linux guests. A workaround is to disable mouse integration if you can’t select any text by dragging in one of these browsers (Host+I).